1. Field of the Invention
The invention relates to a capacitive proximity switch for detecting the change in the capacitance relative to a setpoint capacitance Cset by the approach or retreat of an object in the sensitive area of a proximity switch, especially for use in the door handle of a motor vehicle, with an electrode system which has at least one response electrode and with a switching output, the measured value which corresponds to the change in capacitance being compared to an operating threshold S1 and at the switching output a switching signal being produced depending on the measured value.
2. Description of Related Art
As is recognized, capacitive proximity switches respond when the capacitance between the response electrode and a reference electrode reaches or exceeds a certain value, specifically because the response electrode approaches an object which for its part is galvanically or capacitively connected to the reference electrode. Often the frame or ground potential acts as a reference electrode. Here, the capacitive proximity switch detects the change of capacitance relative to a setpoint capacitance and produces a corresponding measured value. However, since generally such a small change of capacitance is not designed to lead to an output signal, the measured value is compared to an operating threshold S1 so that only then is an output signal, i.e., a switching signal, produced when the change in capacitance or the corresponding measured value is larger than the operating threshold.
The measured value can be, for example, a voltage. However, the measured value can also be other quantities which are used as a measure for the change in the capacitance. For example, the measured value can be a number of clocked discharge processes of a reference capacitor or a discharge interval. Then, a quantity is chosen as the operating threshold which corresponds to the quantity which forms the measured value; i.e. a voltage, a number discharges or a discharge interval.
Often, the presence of a certain object is to be detected with the capacitive proximity switch. Then the problem can arise that a change in capacitance is measured which is greater than the operating threshold, but does not originate from the object which is actually monitored—the target object—but from another object—a false object.
This problem can, in any case, theoretically be solved by the operating threshold being set so high that the probability is very low that a change in capacitance which has been caused by a false object or the corresponding measured value is greater than the operating threshold. This measure can suppress most of the possible errors, but the choice of a high operating threshold, however, clearly reduces the sensitivity of the capacitive proximity switch at the same time.
Although this invention can be used quite generally in any capacitive proximity switch in which both high sensitivity and also good noise suppression are desirable, the invention is described below, without being limited to a specific application example, is described specifically relative to a capacitive proximity switch which is used in the door handle of a motor vehicle and serves there for unlocking and locking the door lock.
The advantage of the use of a capacitive proximity switch to unlock (or also to lock) the door handle of a motor vehicle consists in that the desired actuation is necessary, but it is sufficient when the individual approaches the sensitive area of the motor vehicle in a purposeful manner. Of course, the door handle of the measured value is the sensitive area. (When it is a matter of locking or unlocking the trunk of a measured value, the sensitive area for dedicated actuation can also be the actuating handle on the trunk lid. The expression door handle is always used below. Everything which is described in conjunction with the expression door handle of course also applies with reference to the actuating handle on the trunk lid).
From the early days of the motor vehicles through the 1970s, the vehicle owner had hardly any other possibility than locking or unlocking and opening his motor vehicle with a mechanical key. Due to the increasing use of electronics in motor vehicles in the 1980s, centralized locking and unlocking, and afterwards, remote controls began to increasingly appear in motor vehicles. In North America, radio remote controls, and in Europe, infrared remote controls predominate. For some years, a so-called smart card system has been known (see, “Siemens-Zeitschrift”, 1/96, pages 32–35) in which in the motor vehicle there is an identification receiver. To gain access to the motor vehicle, i.e., especially to lock and unlock the door lock, the individual must carry an identification sender. Since the identification sender has the shape and size of a credit card, it is very easy for the individual to carry this card. By an authorization query proceeding from the identification receiver to the identification sender, the individual who is carrying the identification sender can be identified as an “authorized” individual.
If, at this point, the response electrode of the capacitive proximity switch is on the door handle or integrated into the door handle, as is known, for example, from German Patent Application DE 196 17 038 A1, it can happen that external effects can lead to the capacitive proximity switch triggering in an unwanted manner—for example, by rain, snow or ice allowing the capacitance between the response electrode and the reference electrode to become great enough. German Patent Application DE 196 20 059 A1 and corresponding U.S. Pat. No. 5,880,538 disclose a capacitive proximity switch which is made such that it switches only when the capacitance between the response electrode and the reference electrode changes at a rate which is greater than a lower boundary value. If the capacitance between the response electrode and the reference electrode changes at a rate which is smaller than the lower boundary value, the proximity switch does not switch, not even when the capacitance between the response electrode and the reference electrode reaches a relatively great value.
In the circuit arrangement known from German Patent Application DE 196 20 059 A1 and U.S. Pat. No. 5,880,538 for unlocking at least one door lock of a motor vehicle with a dynamically operating capacitive proximity switch, the above described evaluation of the rate of the change of capacitance can suppress noise effects which lead to an essentially static change in capacitance.
However, in addition, dynamic noise effects can occur which then also lead to a dynamic change in capacitance so that, in the known circuit arrangement or in the known capacitive proximity switch, they do not lead to a switching signal only when the operating threshold of the capacitive proximity switch is correspondingly high.
In particular, for the application of a capacitive proximity switch described here by way of example, to unlock the door lock of a motor vehicle, however with respect to the speed of the capacitive proximity switch, the requirements are relatively high. The approach of the hand of an individual to the door handle is designed to cause unlocking of the door handle, to open the door as in the past the door handle having to be actuated. This results in the requirement that the door lock must already be unlocked before the user actuates the door handle. Moreover, since the door lock of a motor vehicle is not to be unlocked by just any individual who approaches the door handle with his hand, before unlocking the door lock, generally the access authorization of the user is checked. This can take place, for example, by means of the above described smart card system. The above described requirements yield the necessity of a lead time which is on the order of 100 ms for normal door locks.
So that both the access authorization of the user can be checked and also the door lock can be unlocked within this lead time, either very rapid data transmission of the authorization query and very prompt reversal of the door lock between the locked state and the unlocked state or very high sensitivity of the capacitive proximity switch is necessary. To achieve a high transmission rate of data communication and prompt reversal of the door lock, both a high-quality and thus expensive transmission system and also a high-quality door lock are necessary.
On the other hand, if the required lead time is relatively large since, for example, the unlocking mechanism of the door lock is relatively slow-acting, the capacitive proximity switch must be set to a very high sensitivity so that the required lead time can be maintained. However, this leads to changes in capacitance also being registered by the proximity switch which are caused by false objects. The false objects which cause unwanted dynamic changes in capacitance can be especially raindrops.
In the application of a capacitive proximity switch described here for locking and unlocking the door lock of a motor vehicle, this can lead to unacceptable consequences. When using the capacitive proximity switch in the locking system of a motor vehicle, the triggering of the authorization query by the identification receiver is associated with the switching process, i.e., with the presence of a corresponding switching signal, of the proximity switch. The activation of the identification receiver causes increased current consumption which loads the battery of the motor vehicle. When during a longer lasting rain, the capacitive proximity switch relatively often ascertains a change in capacitance which leads to a switching signal each time, the resulting frequent authorization query of the identification receiver can lead to undue discharge of the motor vehicle battery.